Monday, November 03, 2014

Functional and Reactive Domain Modeling

The first time I was formally introduced to the topic was way back when I played around with Erik Evans' awesome text on the subject of Domain Driven Design. In the book he discusses various object lifecycle patterns like the Factory, Aggregate or Repository that help separation of concerns when you are implementing the various interactions between the elements of the domain model. Entities are artifacts with identities, value objects are pure values while services model the coarse level use cases of the model components.

Traditionally we followed the recommendations of Erik in our real world implementations and used the object oriented paradigm for modeling all interactions. We started talking about rich domain models and anemic domain models. The rich model espoused a richer agglomeration of state and behavior within the model, while the anemic model preferred to keep them decoupled. Martin Fowler sums this up in his post on Anemic Domain Models ..

"The basic symptom of an Anemic Domain Model is that at first blush it looks like the real thing. There are objects, many named after the nouns in the domain space, and these objects are connected with the rich relationships and structure that true domain models have. The catch comes when you look at the behavior, and you realize that there is hardly any behavior on these objects, making them little more than bags of getters and setters. Indeed often these models come with design rules that say that you are not to put any domain logic in the the domain objects. Instead there are a set of service objects which capture all the domain logic. These services live on top of the domain model and use the domain model for data."

Go Functional

In Functional and Reactive Domain Modeling I look at the problem with a different lens. The primary focus of the book is to encourage building domain models using the principles of functional programming. It's a completely orthogonal approach than OO and focuses on verbs first (as opposed to nouns first in OO), algebra first (as opposed to objects in OO), function composition first (as opposed to object composition in OO), lightweight objects as ADTs (instead of rich class models).

The book starts with the basics of functional programming principles and discusses the virtues of purity and the advantages of keeping side-effects decoupled from the core business logic. The book uses Scala as the programming language and does an extensive discussion on why the OO and functional features of Scala are a perfect fit for modelling complex domains. Chapter 3 starts the core subject of functional domain modeling with real world examples illustrating how we can make good use of patterns like smart constructors, monads and monoids in implementing your domain model. The main virtue that these patterns bring to your model is genericity - they help you extract generic algebra from domain specific logic into parametric functions which are far more reusable and less error prone. Chapter 4 focuses on advanced usages like typeclass based design and patterns like monad transformers, kleislis and other forms of compositional idioms of functional programming. One of the primary focus of the book is an emphasis on algebraic API design and to develop an appreciation towards ability to reason about your model.

Go Reactive

The term "reactive" has recently become quite popular in describing systems that are responsive, scalable and adaptive. In implementing complex domain models, we find many areas which can be made more responsive by implementing them as non blocking operations instead of the standard blocking function calls. Using higher level concurrency primitives like actors, futures and data flow based computing we can compose asynchronous operations and increase the net throughput of your model. The second part of the book discusses how you can combine the principles of functional programming with the reactive way of implementing behaviors. The paradigm of application design using the principles of functional programming together with an asynchronous non-blocking mode of communication between the participating entities promises to be a potent combination towards developing performant systems that are relatively easy to manage, maintain and evolve. But designing and implementing such models need a different way of thinking. The behaviors that you implement have to be composable using pure functions and they can form building blocks of bigger abstractions that communicate between them using non-blocking asynchronous message passing.

Functional meets Reactive

Functional and Reactive Domain Modeling takes you through the steps teaching you how to think of the domain model in terms of pure functions and how to compose them to build larger abstractions. You will start learning with the basics of functional programming principles and gradually progress to the advanced concepts and patterns that you need to know to implement complex domain models. The book demonstrates how advanced FP patterns like algebraic data types, typeclass based design and isolation of side-effects can make your model compose for readability and verifiability. On the subject of reactive modeling, the book focuses on the higher order concurrency patterns like actors and futures. It uses the Akka framework as the reference implementation and demonstrates how advanced architectural patterns like event sourcing and command-query-responsibility-segregation can be put to great use while implementing scalable models. You will learn techniques that are radically different from the standard RDBMS based applications that are based on mutation of records. You’ll also pick up important patterns like using asynchronous messaging for interaction based on non blocking concurrency and model persistence, which delivers the speed of in-memory processing along with suitable guarantees of reliability.

Looking forward to an exciting journey with the book. I am sure you will also find interest in the topics that I discuss there. And feel free to jump on to AuthorOnline and fire questions that we all can discuss. I am sure this will also lead to an overall improvement in the quality of the book.